Austenitic stainless steel



Patented July 20, 1954 UNITED STATES ATENT OFFICE AUSTENITIC STAINLESSSTEEL Robert H. Henke, Pittsburgh, Pa., assignor to N Drawing.

8 Claims.

This invention relates to steel and in particular to austenitic chromiummanganese steel.

Austenitic chromium manganese steel has been known for a number ofyears. Thus it has been known that steels containing up to about 13%chromium are completely austenitic if they .contain 14% or highermanganese. However, with higher chromium contents it is found thatferrite is present in the structure and that it is not possible toprevent the formation of ferrite by increasing the manganese content.Instead steel containing 15% chromium, 12% manganese and up to 1.5%nickel has been produced as having an austenitic structure, it beingnoted that where the chromium content was increased, the manganesecontent was decreased and nickel was employed to maintain the austeniticstructure.

In view of the present shortage of nickel, the chromium manganese steelhas become attractive as a substitute for the well-known 18% chromium-8%nickel stainless steel. Prior attempts to produce such substitutes; forexample, 13% chromium-14% manganese, or 16% chromium- 14% manganese-1%nickel, or 17% chromium- 12% manganese-2% nickel, or 17% chromium- 9%manganese-3% nickel, have resulted in the production of steel havingmechanical properties similar to type 301 steel, but such steel cannotbe satisfactorily produced and rolled directly from ingots larger than 9inches square. fore desirable while producing a substitute for thewell-known 18% chromium-8% nickel stainless steel to improve thecorrosion resistance characteristics, the mechanical properties and theweldability and bendability of the known chromium manganese steels,while at the same time produce a steel that has an austenitic structureand is capable of being hot worked directly from large ingot size toslab form and to thereafter be capable of being cold worked.

An object of this invention is to provide a cold workable austeniticchromium manganese steel.

Another object of this inv'entionis to provide an austenitic steelcontaining 15 to 17.25% chromium, 15 to 19% manganese and not more than2% nickel that is workable by a single conversion heat treatment fromlarge ingot size to slab form and is thereafter capable of being coldworked to stripform havinggoodphysical properties and resistancetocorrosioni;

It is there- Application November 20, 1952, Serial No. 321,731

Other objects of this invention will become apparent from the followingdescription.

The steel of this invention is an austenitic steel capable of beingdirectly converted by hot rolling from large ingot size into slab form.Fundamentally the steel comprises a high manganese, high chromium steelcontaining from a small but essential amount up to 2% nickel, the nickelcooperating therein to maintain an austenitic structure. The alloyingelements in the broad aspect of the invention comprise 15 to 19%manganese, '15 to 17.25% chromium, not more than 2% nickel, not over.20% carbon, not over 1% silicon, not over .04% of each of phosphorusand sulphur, .05 to 25% nitrogen, .003 to 15% of rare earth element andthe balance iron. All of the compositions given herein and in the claimsare by weight.

A preferred embodiment of the invention is a steel comprising 16 to 18%manganese, 15 to 16% chromium, not more than about 1.25% nickel, notover 20% carbon, not over 1% silicon, not over 030% of each ofphosphorus and sulphur, .05 to 25% nitrogen, .003 to 15% rare earthelement and the balance iron.

Where the chromium content is in the indicated range of 15 to 17.25%with the manganese in the high range of 15 to 19%, it is found thatunless nickel is present in amounts up to 2%, the resulting steels haveferrite present in the as cast structure in amounts which render thesteel undesirable for hot working directly from ingot size to slab form.Thus where from 10 to 25% ferrite with the balance austenite is formedin the steel, as is the case where nickel is not present, the austeniteis usually surrounded by the ferrite with the result that as thetemperature decreases during the hot rolling processing,the austenitebecomes hard and the plastic deformation of the steel ingot is such thatit exceeds the ductility limits of the ferrite resulting in ruptures.

Even where nickel in small amounts of up to 2% is included as anessential element to cooperate in producing a predominantly austeniticstructure, it has been found substantially impossible to directly hotroll the ingot to the slab form unless from .003 to .15% of rare earthelement of the lanthanide series such as cerium,

lanthanum, neodymium and similar rare earth elements is present. Inpractice, the rare earth element is usually added to the melt in theform of misch metal in which cerium and lanthanum predominate. While theterm rare earth element is employed in this description and in theclaims in the singular form, it is to be understood that such expressionincludes either one of the rare earth elements such as cerium ormixtures thereof such as cerium and/or lanthanum with the other knownrare earth elements, the latter usually being the case since the rareearth elements are now commonly available in the open market as packagedcompounds identified under the trade names of T-Conipound, L-Metal, andLan- Ger-Amp.

As examples of the eifect of the presence of the rare earth element asan essential alloying element on the austenitic chromium manganese steelof this invention, reference may be had to the following table ofanalysis of the composition of diiferent heats made while developing thepresent steel.

4 austenite appeared to resist plastic deformation. The other ingotslisted in the foregoing table and containing the rare earth element asindicated were successfully directly hot worked from Composition,Percent by Weight Heat No.

C Mn P S Si Cr Ni N2 large ingot size, as for example, 16 by 31", toslabs 3 inches or less in thickness without any accompanying detrimentalcracking. The best hot rolled results are obtained Where the steel has achromium content of from 15 to 17% and manganese content of 16 to 18%with up to 1.25%

nickel, the higher manganese and lower chromium contents rendering thesteel more completely austenitic in the as cast condition. However, inall cases there is some ferrite in the structure as cast" and usually inthe slab, such ferrite however tending to disappear during the annealingof the cold rolled strip. The steel as thus finally processed cantherefore be termed an austenitic steel since the ferrite disappears 20in the steel as worked into strip.

As an example of the physical properties obtained with the steel of thisinvention, reference Total Ce Rare Earths None None None None None NoneNone None The heats 88880, 88884, 98070, and 98080 of the foregoingtable and having a composition within the range given hereinbefore,except that such heats are devoid of the presence of the rare earthelement, are not commercial since it has been found that they cannot besuccessfully hot rolled directly from large ingot size to slab form.Such heats can only be formed into slab form by expensive double andtriple conversion heat treatments resulting in yields of only about ofthe metal.

Where such steels are processed in an effort to'hot roll them directlyfrom the ingot to slab form, it is found that the slabs, are ruined asby edge checking and cracking long before the ingot is processed to adesired slab thickness of about 3 inches or less. Thus, for example, inrolling an ingot 16" by 31" of heat 88880 from a temperature of 2250 to2300 F. after soaking at 2200 F. for 2 hours, rolling had to be stoppedat approximately 6 inches thickness because of edge cracking. Similarly,hot rolling of a 16" by 31 ingot of heat 8888 1 had to be discontinuedwhen rolled to an 8" by 22 slab size because of cracking. Similarresults were obtained in rolling ingots of heats 98070 and 98080. In allcases hot rolling failure took place in an unusual manner in that inattempting to roll the ingots, they appeared to be entirely satisfactoryuntil reduced to approximately 8" in thickness. At this point, that isin the vicinity of the 8 inch slabbing pass, the metal tore along theedges of the surface all at once as if the metal were hot short.However, a micro-examination of the metal so treated indicated thatinstead of the metal being hot short,

a temperature had been reached at which the may be had to the steel ofheat 77834 identified in the foregoing table. A slab rolled directlyfrom a 16" by 31" ingot to 3 inch slab as described hereinbefore, whenfurther hot rolled from a temperature of 2225 to 2300 F. to a strip .090inch thick and annealed at a temperature of 1850 to 2000 F. had thefollowing properties:

Yield strength (02% ofiset) 39,830 p. s. i. Tensile strength 100,100 p.s. i Percent elongation 64.0 Rockwell B Hardness 82-84 When thereaftercold rolled to .078 inch, the strip had the following properties:

Yield strength (02% offset) 65,960 p. s. i. Tensile strength 110,750 p.s. i. Per cent elongation 53.0

Rockwell B Hardness 93-95 When such strip was thereafter annealed at1850 to 2000 F. and then cold rolled to a thickness of .062", theresulting strip had the following properties:

Yield strength (02% offset) 113,150 10. s. i. Tensile strength 142,600p. s. i. Per cent elongation 21.0

Rockwell C Hardness 29-31 Thereafter when again annealed at 1850 to 2000F., further cold rolled to a thickness of .040 inch and annealed at 1850to 2000 F., the following properties were obtained:

Yield strength (0.02% offset) 43,000 p. s. i. Tensile strength 99,950 p.s. i. Per cent elongation in 2" 58.0

Rockwell B Hardness 83-85 The mechanical properties for the steel ofthis invention as exemplified by heat 77834 are thus seen to closelyduplicate the mechanical properties of the well-known type 301 stainlesssteel very closely with the strength and rate of work hardening tendingto be on the low side of the type 301 average values. However, itsability to be fabricated by bending, forming and welding appears to beequal to that of the type 301 stainless steel.

As a further example of the tensile strength and physical propertiesobtained with a similar steel, reference may be had to heat 98027 whichwhen processed directly from ingot to slab form as describedhereinbefore and then annealed and cold worked to a thickness of .044inch, had the following physical properties:

Yield strength (2% ofiset) 124,500 p. s. i. Tensile strength 148,500 p.s. 1. Per cent elongation in 2" 18.5 Rockwell C Hardness 35.5 Bend135-3/16" R, 0K

On the other hand, heat 98021 having the lower manganese content of16.06%, but a higher chromium content of 17.25% and a higher nickelcontent of 1.83%, when cold rolled to .078 inch and annealed at1850-2000 F., had the following tensile and hardness properties:

Yield strength (.2% offset) 47,700 p. s. i. Tensile strength 99,760 p.s. i. Per cent elongation in 2" Rockwell B Hardness 83-84 All of thesteels of this invention have adequate corrosion resistance so that theycan be used in applications where the well-known type 430 stainlesssteel has been used and further can be employed in other applicationswhere the type 430 stainless steel is unsatisfactory because ofmechanical properties or physical properties. As an example of thecorrosion resistance of the steels of this invention containing up to 1%nickel as exemplified by heat 77834 in unwelded annealed cold rolledstrip, heat 77834 was subjected to the Huey test in boiling nitric acidand the weight losses of five 48 hour periods were converted to inchespenetration per month and averaged with the result that the tests show arate of attack in the range of .005-.006 inch I penetration per month.While these results are not quite as good as the rate of attack of type301 stainless steel, nevertheless, they compare quite favorably with therate of attack of type 430 stainless steel. On the other hand, when thesteels having the higher nickel content, as for example heat 98021, weresubjected to the same test, the rate of attack was in the range of003-0035 inch penetration per month. It is thus seen that with thehigher nickel content, the resulting steel has a better corrosionresistance than the steels containing up to 1% nickel. Such rate ofattack for the steels containing from 1 to 2% nickel is better than therate of attack shown by type 430 stainless steel and compares favorablywith the rate of attack of less than .002 inch penetration per month forthe well-known types 301 and 302.

The steels of this invention and in particular the steels containing upto 1% nickel and with from .08 to .12% carbon can be readily welded byhand arc welding using a filler rod of the same composition or a fillerrod of type 308 steel as well as by the heliarc method or resistancewelding, such welding results being equivalent to the .6 welding resultsobtained on type 301 or type 302 stainless steel. Further, in order todetermine the resistance to intergranular corrosion, fusion welded lightgauge sheet and strip formed of the steel of this invention have beensubjected to the well-known Krupp test for 48' hours and have been foundto bend satisfactorily after such test without any evidence ofintergranular corrosion around the weld. Likewise, the steel of thisinvention containing from 1 to 2% nickel has also been welded andsubjected to the Krupp tests for a 48 hourperiod. The steel containingthe higher nickel content has been found to be satisfactory in that itshows a resistance to intergranular corrosion similar to type 302.

The steel of this invention has proven to be an austenitic steel ofacceptable corrosion resistance with a minimum nickel content. From theweldability, bendability, corrosion resistance and physical and tensileproperties, it can readily be substituted for the well-known 18%chromium- 8% nickel stainless steels. A yield of 75% or higher isobtained in working the steel from the large ingot size to slab form.Further the composition is readily controlled and can therefore bereadily reproduced by anyone skilled in the art.

I claim:

1. An austenitic steel consisting essentially of 15% to 19% manganese,15% to 17.25% chromium, not more than 2% nickel, not over 20% carbon,not over .040% of each of phosphorus and sulphur, .05% to .25% nitrogen,003% to .15% rare earth element of the lanthanide series, and thebalance iron, the steel having the characteristic of being hot workabledirectly from large ingot size to slab form.

2. An austenitic steel consisting essentially of 16% to 18% manganese,15% to 16% chromium, not more than 1.25% nickel, not over .12% carbon,not over 1% silicon, not over .030% of each of phosphorus and sulphur,.05% to .25% nitrogen, .003% to .15% rare earth element of thelanthanide series, and, the balance iron, the steel having thecharacteristic of being hot workable directly from large ingot size toslab form.

3. An austenitic steel consisting essentially of 16% to 18% manganese,15% to 16% chromium, 1.25% to 2% nickel, not over .20% carbon, not over1% silicon, not over 030% of each of phosphorus and sulphur, .05% to.25% nitrogen, .003% to .15% rare earth element of the lanthanideseries, and the balance iron, the steel having the characteristic ofbeing hot workable directly from large ingot size to slab form.

4. An austenitic steel consisting of about 16.00% manganese, about17.25% chromium, about 1.83% nickel, about .095% carbon, about .35%silicon, about .024% phosphorus, about 01% sulphur, about .11% nitrogen,about .023% rare earth element of the lanthanide series, and the balanceiron.

5. An austenitic steel consisting of about 17.59% manganese, about15.32% chromium, about .95% nickel, about .10% carbon, about 38%silicon, about .022% phosphorus, about 008% sulphur, about .13%nitrogen, about .022% rare earth element of the lanthanide series, andthe balance iron.

6. An austenitic steel composed of 15% to 19% manganese, 15% to 17.25%chromium, not more than 2% nickel, not over .20% carbon, not over 040%of each of phosphorus and sulphur, .05% to .25% nitrogen, .003% to .15%of rare Z earth element selected from the lanthanide series, and thebalance ircn.

'7'. An austeni-tic steel composed of 15% to 19% manganese, 15% to17.25% chromium, not more than 2% nickel, not over .20.% carbon, notover 1% silicon, net over .040% of each of phosphorus and sulphur, 05%to 25% nitrogen, .003% to .15% of rare earth element residue from mischmetal addition, and the balance iron.

8. An austenitic steel composed of 15% to 19% manganese, 15% to 17.25%chromium, not more than 2% nickel, not over 20% carbon, not overReferences Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,553,330 Post et a1. May 15, 1951 2,616,798 Zeigler et a1Nov. 14, 1952

1. AN AUSTENITIC STEEL CONSISTING ESSENTIALLY OF 15% TO 19% MAGANESE,15% TO 17.25% CHROMIUM, NOT MORE THAN 2% NICKEL, NOT OVER .20% CARBON,NOT OVER .040% OF EACH OF PHOSPHOROUS AND SULPHUR, .05% TO .25%NITROGEN, .003% TO .15% RATE EARTH ELEMENT OF THE LANTHANIDE SERIES, ANDTHE BALANCE IRON, THE STEEL HAVING THE CHARACTERISTIC OF BEING HOTWORKABLE DIRECTLY FROM LARGE INGOT SIZE TO SLAB FORM.